1. Field of the invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
2. Description of the Related Art
Air-fuel ratio control apparatus for an internal combustion engine are known which increase the temperature of a three-way catalytic device disposed in an exhaust passage of the engine to an activation temperature. That is, in an early period after the engine is started, such systems warm up a three-way catalytic device by supplying residual unburned fuel and oxygen to the three-way device to cause them to react around the three-way device. An example of this type of air-fuel ratio control apparatus for an internal combustion engine is disclosed in Japanese Patent Application Laid-open No. SHO 62-178740. An air-fuel ratio control apparatus for a multi-cylinder internal combustion engine described in this laid-open patent application warms up a three-way catalytic device by performing fuel-rich operation in one cylinder to supply unburned fuel to the three-way catalytic device while performing fuel-lean operation in another cylinder to supply unburned oxygen to the three-way catalytic device.
However, air-fuel ratio control in the apparatus described in Japanese Patent Application Laid-open No. SHO 62-178740 is complicated, since it is necessary to perform fuel-rich operation in one cylinder while performing fuel-lean operation in another cylinder, that is, performing air-fuel ratio control separately for the individual cylinders, during the warm-up of the three-way catalytic device.
An air-fuel ratio control apparatus for an internal combustion engine capable of warming up a three-way catalytic device by supplying unburned fuel and oxygen to the three-way catalytic device without a need to perform different air-fuel ratio controls for the individual cylinders is described in, for example, Japanese Patent Application Laid-open No. HEI 5-33705. The internal combustion engine air-fuel ratio control apparatus described in this laid-open patent application warms up a three-way catalytic device by forcibly switching between fuel-rich operation for supplying unburned fuel to the three-way catalytic device and fuel-lean operation for supplying unburned oxygen to the three-way catalytic device at predetermined time periods.
However, in the internal combustion engine air-fuel ratio control apparatus described in Japanese Patent Application Laid-open No. HEI 5-33705, the air-fuel ratio control is still complicated, since it is necessary to switch between fuel-rich operation and fuel-lean operation at every predetermined time period during the warm-up of the three-way catalytic device.